THE LH WING PITOT TUBE AND WIRING
As I progressed along on the left wing construction, I began to think about what type of Pitot/Static system I wanted to install. The plans call for the most basic pitot tube - a piece of bent aluminum tubing attached to the wing skin. Other builders have used a standard heated pitot such as the type found on most IFR equipped small planes. I have owned two single engine Pipers and they have a heated pitot tube with the static port built into it. I decided to go this route. I ordered a P/N 96392-05 (Piper part number) from API in Memphis Tennessee. They were the Piper distributor in the area. It listed for $325.00. I started the installation of the pitot tube on 12-21-03. The photo at left below shows the tube installed just outboard of the outboard wing access panel. It is centered between the next two ribs outboard of the access panel. This location is one bay further out than the plans call for but it gets the pitot tube out of the way of the aileron bellcrank. The plumbing is another story as you will see later. The photo below right shows a doubler that I made to beef up the wing skin due to the hole I put in it. I made the doubler out of .032 2024 T3 aluminum which is one gauge thicker than the wing skin.
The photo at left (below) shows the doubler cleco'd to the back side of the wing skin. Notice the joggle that I put in it. The doubler gets riveted to the inside of the spar horizontal leg. The joggle allows the doubler to go around to the other side of the spar without a noticeable lump. After I cleco'd the skin back on with the doubler in place, I attached the pitot tube again. I felt that the skin needed another stiffener because the pitot tube still was still too easy to move around. I made an "L" angle stiffener to be attached to the aft edge of the doubler. It also would attach to the rib on each side of the pitot tube bay. Photo at right (below) shows the stiffener I made cleco'd in place. I made the stiffener from .025 2024 T3. My thinking on this stiffener was that if the pitot tube was allowed to vibrate back and forth easily, the wing skin would eventually crack. This stiffener made the tube mount much more resistant to movement.
After the parts were made it was time to fabricate the plumbing to the pitot tube. The pitot tube came with 3/32" inside diameter rubber hoses attached that were approximately 12" long. So that I did not get confused about which line was pitot and which was static after I had them plumbed to the inside of the fuselage I made the aluminum line that connected the static line to the airplane out of 1/4" outside diameter tubing. The photo below left shows my first attempt at making the static line. I put bends in it to go below the aileron bellcrank mount. After thinking about that for awhile, I decided that water could collect in the low spot. I removed this line and made another one. The photo at right below shows my second attempt. This time I made the static line a direct shot right across the top of the lower aileron bellcrank mounting bracket. Then I added the pitot line which I made from 3/32" O.D. aluminum tubing.
The lines penetrate each rib through existing holes which were enlarged as required to accept snap bushings P/N 04-05738 For the 1/4" line and P/N 04-05710 for the 3/32" line. These snap bushings came from Aircraft Spruce and Specialty in Atlanta, GA. Well, it was strike 2 for the static line. I installed the aileron bellcrank and it came in contact with the static line before it moved far enough to get the to the spar. The picture at left below shows my final fabrication of this line. It still has a low spot but not as severe as it was the first time I made the line. As you can see in the photo, I terminated the two lines at the wing rib just prior to the pitot tube mount. I used P/N AN832 bulkhead fittings with P/N AN924 nuts (-3D and -4D) to attach the lines to the rib. The photo below right shows a closer look at the two lines. It also shows that I have started to run electrical wires in the wing at this time. The one that ends in this bay is for pitot heat. The large grey wire is actually a shielded triple wire that is going to power the wing tip strobe light. The other two wires are for the wing tip nav light and the landing light.
The photo above left shows the pitot tube stiffener and doubler riveted to the ribs and to the spar at the leading edge attach rivets. It also shows a series of snap bushings with nothing in them. These are for a coax cable that will run to an antenna that I am going to put in the wing tip. It will either be a nav or a comm antenna. I have heard that a comm antenna mounted in the tip could have marginal performance. I have more research to do. If it looks like a comm antenna will work here then I will mount a nav antenna in the other wing tip. The right hand picture shows the wires as they extend into the wing tip area. The light brown wire that is still on the spool is the coax cable for either the comm or the nav antenna.
The photo below left shows another shot of the aileron bellcrank area. The wire that is attached to the rib via the screw that has not been tightened yet is the ground wire for the heater in the pitot tube. Trust me. I tightened the screw after I took the picture.
The picture above right shows how I attached the flexible rubber hoses from the pitot tube to the aluminum lines. (The pitot tube is not fastened down yet in the photo). For the pitot line I cut a piece of 3/32" aluminum line about 3" long and flared one end. I put a P/N AN818-3D "B" nut and a P/N AN819-3D sleeve on this end and attached it to the outboard end of the -3 bulkhead fitting. After the "B" nut was tight I cut the pitot hose to length and slid it over this line. I made another line about 3" long out off 1/4" aluminum tubing. I flared one end. I slid two P/N AN818-4D "B" nuts and two P/N AN819-4D sleeves into place. I flared the other end and attached it to the ¼ bulkhead fitting. I attached a P/N AN919-2D reducer union to the other end. I then made another 3" long 3/32" diameter tube identical to the one I used on the pitot line. I attached this line to the reducer union at the outboard end. I then cut the flexible static line from the pitot tube to length and slid it over this line. The pitot tube is now plumbed and the wires are in the wing. The lower wing skins can now be installed.